Some humans perceive the taste of phenylthiocarbamide (PTC) and its chemical relative, propylthiouracil (PROP) as intensely bitter at low concentrations, while others find it undetectable, even at much higher concentrations. Although this human trait has been recognized for a long time, and its Mendelian pattern of inheritance documented, the corresponding gene is unknown. Preliminary analysis of 100 families conducted by the P.I. suggests that the gene may reside on the short arm of chromosome 5. Therefore, the P.I. proposes to conduct experiments to precisely map and identify the gene. She plans to test the linkage between chromosome 5p and taster status in an additional 100 newly collected families (Specific Aim 1). This would provide a replicate sample to verify the initial map location, and the pooled sample of 200 families will be sufficiently powerful to meet the genome-criteria of significant linkage. In Specific Aim 1, the P.I. proposes to densely genotype these 200 families using markers in the 5p13 region, construct haplotypes, and find those haplotypes shared among tasters, but not shared among nontasters. The linked area of 5p contains 26 simple sequence repeats and single nucleotide polymorphisms within a 13MB region, which will be used for this dense mapping. In Specific Aim 3, the P.I. will initiate an evaluation of candidate genes located within the region identified, by screening for variabilities associated with PROP-taster status.